2.3 Oxygen Therapy Devices & Delivery

Oxygen Therapy Devices & Delivery

Oxygen therapy is the most common respiratory intervention, and the TMC Examination rewards an RT who can pick the right device, estimate the delivered FiO2, and titrate it to a target SpO2. The organizing concept is the split between low-flow systems, whose delivered concentration drifts with the patient's breathing, and high-flow systems, which lock in a fixed FiO2.

Low-Flow (Variable FiO2) Devices

Low-flow systems supply less gas than the patient's total inspiratory demand, so room air is pulled in to make up the difference. A faster, deeper breath dilutes the oxygen and lowers FiO2; a slow, shallow breath raises it.

Nasal cannula estimation: room air is 21%, and each liter adds about 4%. So 1 LPM is ~24%, 2 LPM ~28%, 3 LPM ~32%, 4 LPM ~36%, 5 LPM ~40%, and 6 LPM ~44%. These are estimates that shift with respiratory rate and tidal volume.

High-Flow (Fixed FiO2) Devices

High-flow systems deliver total flow that meets or exceeds peak inspiratory flow, so essentially no room air is entrained beyond what the device itself controls, and the FiO2 is precise.

Venturi (air-entrainment) mask: Delivers exact concentrations — commonly 24%, 28%, 31%, 35%, 40%, and 50% — using color-coded jet adapters. Oxygen forced through a narrow jet speeds up and drops its lateral pressure (Bernoulli principle), entraining a fixed ratio of room air. Lower FiO2 jets entrain more air, so they produce higher total flow. The Venturi is the device of choice when a COPD patient needs a precise, controlled low concentration.

High-flow nasal cannula (HFNC): Delivers heated, humidified gas at flows up to 60 LPM with FiO2 blended from 21% to 100%. It generates a mild positive airway pressure (~1 cmH2O per 10 LPM), washes CO2-rich gas out of the anatomical dead space, and is generally better tolerated than a tight CPAP mask. Indications include acute hypoxemic respiratory failure, post-extubation support, and pre-oxygenation before intubation.

Oxygen Titration in COPD

Chronic CO2 retainers warrant a target SpO2 of 88-92%, not 95-100%. Excess oxygen worsens ventilation/perfusion matching (releasing hypoxic pulmonary vasoconstriction and the Haldane effect) and can blunt the hypoxic ventilatory drive, producing hypoventilation and CO2 narcosis. Use the lowest FiO2 that holds 88-92%, favor a Venturi mask for precision, and recheck arterial blood gases after any change. Critically, never withhold oxygen from a genuinely hypoxemic patient — titrate, do not deny.

The P/F Ratio

The P/F ratio (PaO2 divided by FiO2 as a decimal) rapidly grades oxygenation and frames the Berlin definition of ARDS.

Worked example: PaO2 80 mmHg on FiO2 0.40 gives 80 / 0.40 = 200, sitting at the ARDS threshold. PaO2 60 on FiO2 0.50 gives 120, squarely in the moderate-ARDS band and a patient who usually needs PEEP and mechanical ventilation.

Reservoir-Mask Troubleshooting

Reservoir masks fail in predictable, testable ways. On a non-rebreather, the bag should remain at least one-third inflated through inspiration; if it collapses with each breath, the flow is too low for the patient's demand — increase the liter flow rather than swapping devices. The NRB's one-way valves are the safety feature and the failure point: an exhalation-port valve that is missing or stuck open lets room air dilute the FiO2, and an inspiratory valve stuck closed blocks inflow.

The partial rebreather intentionally has no valves, so the patient rebreathes the first third of exhaled gas, which is oxygen-rich dead-space gas rather than CO2-laden alveolar gas; its bag, too, should not fully collapse.

Oxygen Toxicity and Absorption Atelectasis

More oxygen is not automatically safer. Prolonged high FiO2 (commonly cited as above 0.50-0.60 for more than 24-48 hours) generates reactive oxygen species that injure the alveolar-capillary membrane, producing a clinical picture resembling ARDS. A high FiO2 also washes nitrogen out of the alveoli; nitrogen normally splints alveoli open, so its loss promotes absorption atelectasis as the remaining oxygen is rapidly taken up by blood. The practical rule is to use the lowest FiO2 that meets the oxygenation target and to wean it down as the patient improves.

In neonates a separate concern, retinopathy of prematurity, reinforces conservative oxygen targets.

Choosing a Device in Practice

Device selection follows the FiO2 needed and how precise it must be. A mildly hypoxemic ward patient does well on a nasal cannula. A patient needing a reliable moderate concentration but tolerating a mask suits a simple mask or Venturi. A critically hypoxemic patient who is still breathing adequately gets a non-rebreather or high-flow nasal cannula, while a COPD patient needing a tightly controlled low concentration gets a Venturi.

If a device cannot achieve the target SpO2, the answer on the exam is usually to escalate to the next higher-FiO2 or higher-flow system, then to consider noninvasive or invasive ventilation — never to abandon oxygen in a hypoxemic patient.